1. Field of the Invention
The present invention relates to an electrode holder for electric spark machine, more particularly to an improved electrode holder for electric spark machine of the type in which the working electrode is clamped from two directions orthogonal to each other.
2. Description of the Prior Art
Generally, the working electrode of an electric spark machine is fixed in position by an electrode holder secured to a fixture which in turn is mounted to the electrode head of the machine.
The conventional electrode holder consists of an L-shaped support block, an L-shaped bent plate, three bolts and other necessary parts. The corresponding ends of said L-shaped support block and bent plate are joined by screws to form a square frame, and the bolts are threadedly passed through said bent plate at its three selected positions, that is, one bolt is passed through a part of one side portion of said L-shaped bent plate, another bolt through a part of the other side portion of said plate and still another bolt through the bend (middle portion) of sald plate, in a way that the ends of said bolts project out inside the square frame toward the bend (corner) of the opposing L-shaped support block.
The working electrode is positioned at the corner of said support block and said three bolts are tightened so as to hold and clamp the electrode between the corner of said support block and the ends of said three bolts.
According to such conventional electrode holder using bolts as clamping means, however, clamplng of the electrode often proves unstable because the abutting portion of each bolt against the electrode is defined to a limited area of the bolt end. Further, when the bolts are tightened, their ends are pressed against the electrode while turning, so that a force is produced which urges the electrode to lift up from its position abutting against the support block. Consequently, the electrode, which should be vertical or horizontal in correct position, tends to incline slightly against the electrode holder when fixed, resulting in a shift from the correct position of the electrode. Therefore, it was necessary to readjust the tightening of the bolts and correct the fixed position of the electrode by using the positioning mechanism of the electrode holder mounting means while observing the correct position with a meter.
Also, the conventional electrode holder had the problem that since the clamping force of the bolts could vary in each electrode fixing operation, the position of the electrode might be shifted each time the electrode is replaced for conducting rough machining or finish machining or for other purposes, making it necessary to make said time-consuming adjustment of the fixed position each time the electrode is set in position, resulting in a reduced working efficiency.
Further, according to the conventional electrode holder, three bolts are threadedly passed through a bent place at its selected positions and the pressing force of the bolts against the electrode acts always in one direction. However, the thickness of the electrode portion to be clamped is widely diversified, and in case the thickness of the electrode portion to be clamped is either excessively large or excessively small, the direction of pressing force of the bolts against the electrode portion clamped would deviate greatly from the center axis of the clamped electrode portion, making it unable to attain stabilized and secure clamping. Especially in case the electrode portion to be clamped is very small in thickness, the ends of the three bolts would hit against each other before reaching the electrode and cannot perform their normal function, so that, in such a case, there has been no alternative but to place a spacer such as a metal plate between the bolt ends and the electrode as a temporary expedient to effect clamping which could prove unstable.
Thus, it was impossible with the prior art to attain correct and secure clamping of all sizes of electrode with a single set of electrode holder, and for accommodating a variety of electrodes, it was necessary to prepare a plurality of electrode holders different in size.